In techniques for measuring displacement (or velocity) using the principle of interference, such as FMCW (Frequency Modulated Continuous Wave) radar, a standing wave radar, or a self-mixing laser sensor, typically MHP processing such as FFT (Fast Fourier Transform) or an interference fringe counting process, or the like, is used when calculating the displacement or velocity of an object being measured based on the frequency of beats or interference fringes. However, there is a problem in that the establishment of high resolution through FFT requires data over a long sampling interval with a high sampling frequency, requiring excessively large processing time. Moreover, in the process for counting interference fringes, measuring displacements that are less than half a wavelength has required physical vibrations of the sensor, or analysis of the amplitudes of the interference fringes, and thus there has been a problem in that it has only been possible to measure the vibrations that are the periodic motion that is measured, and, additionally, a problem in that the interference fringe counting process has been time-consuming.
On the other hand, the present inventor has proposed a laser measuring device of a wavelength modulation type that uses the self-coupling effect of a semiconductor laser (See Japanese Unexamined Patent Application Publication 2006-313080 (“Patent Document 1”)). The structure of this laser measuring device is illustrated in FIG. 15. The laser measuring device of FIG. 15 includes: a semiconductor laser 201 that emits a laser beam toward an object 210; a photodiode 202 that converts, into an electric signal, the optical power of the semiconductor laser 201; a lens 203 that condenses the beam of the semiconductor laser 201 onto the object 210 and that causes the return light from the object 210 to be incident into the semiconductor laser 201; a laser driver 204 for repetitively alternating between a first oscillating period wherein the oscillating wavelength of the semiconductor laser 201 is increased continuously and a second oscillating period wherein the oscillating wavelength is decreased continuously; a current-voltage converting amplifier 205 for converting into a voltage, and then amplifying, the output current from the photodiode 202; a MHP extracting circuit 206 for taking the second derivative of the output voltage of the current-voltage converting amplifier 205; a counting circuit 207 for counting the number of MHPs (mode hop pulses) included in the output voltage of the MHP extracting circuit 206; a calculating device 208 for calculating the distance to the object 210 and the velocity of the object 210; and a display device 209 for displaying the calculation results by the calculating device 208.
The laser driver 204 provides, to the semiconductor laser 201, as an injection current, a triangle-wave driving current that repetitively rises and falls at a constant rate of change over time. Doing so causes the semiconductor laser 201 to be driven so as to repetitively alternate between a first oscillating period, wherein the oscillating wavelength increases continuously at a constant rate of change, and a second oscillating period, wherein the oscillating wavelength increases continuously at a constant rate of change. FIG. 16 is a diagram illustrating the change in the oscillating wavelength of the semiconductor laser 201 over time. In FIG. 16, P1 is the first oscillating period and P2 is the second oscillating period, wherein λa is the minimum value for the oscillating wavelength in each of the periods, λb is the maximum value for the oscillating wavelength in each of the periods, and Tt is the period of the triangle wave.
The laser beam that is emitted from the semiconductor laser 201 is condensed by the lens 203 to be incident on the object 203. The light that is reflected by the object 210 is focused by the lens 203 to be incident into the semiconductor laser 201. The photodiode 202 converts, into an electric current, the optical power of the semiconductor laser 201. The current-voltage converting amplifier 205 converts into a voltage, and then amplifies, the output current from the photodiode 202. The MHP extracting circuit 206 takes the second derivative of the output voltage from the current-voltage converting amplifier 205. The counting circuit 207 counts the mode hop pulses (MHPs) included in the output voltage of the MHP extracting circuit 206, doing so separately for the first oscillating period P1 and the second oscillating period P2. The calculating device 208 calculates the distance to the object 210 and the velocity of the object 210 based on the minimum oscillating wavelength λa and the maximum oscillating wavelength λb of the semiconductor laser 201 and on the number of MHPs in the first oscillating period P1 and the number of MHPs in the second oscillating period P2. In this way, it is possible to use a self-coupling-type laser measurement device to measure a displacement with a resolution of about one half the wavelength of the semiconductor laser 201, and to measure a distance with a resolution that is inversely proportional to the magnitude of wavelength modulation in the semiconductor laser 201.
The self-coupled laser measuring device enables measurements of displacement and velocity of an object at a higher resolution than when compared to the conventional FMCW radar or standing-wave radar self-mixing laser sensor, or the like. However, because the self-coupled laser measuring device measuring time that is adequate in order to calculate the displacement and velocity, in the same way as with FFT, is required (which, in the example in Patent Document 1 is a half period of the carrier wave for the oscillating wavelength modulation of the semiconductor layer), there is a problem in that there is a calculation error in calculations for objects wherein the changes in velocity are rapid. Moreover, because it is necessary to count the number of MHPs during the MHP period, there is a problem in that it is difficult to achieve a resolution less than a half wavelength of the semiconductor laser.
The present disclosure addresses the problems set forth above, and the object thereof is to provide a physical quantity sensor and physical quantity measuring method able to measure a displacement and a velocity of an object with high resolution, and able to reduce the time required in the measurement.